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        /* 
        双重for循环
        1.我们可以把里面的循环看做是外层循环的语句
        2.外层循环循环一次，里面的循环执行全部
         */
        for (var i = 1; i <= 3; i++) {
            console.log('这是外层循环的第' + i + '次');
            for (var j = 1; i <= 3; j++) {
                console.log('这是内循环的的第' + j + '次');
            }
        }
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